Re: xxx@ourdomain.onmicrosoft.com

Hi BlueBerry88,

Please refer to the following steps to edit the proxyAddresses attribute in your local AD and check the result.

1. Go to Active Directory Users and Computers.
2. Right-click on this user, and click Properties.
3. Click the Attribute Editor tab, find the proxyAddresses attribute.
4. Double click on it, add this onmicrosoft.com SMTP address. After that, force directory synchronization and check if this SMTP address is updated in Office 365.

Thanks,
Bourne Zhang